This invention is related to metal detectors and especially to metal detectors that combine pulse-induction and sine-wave technologies in one instrument.
Metal detectors based on the pulse-induction principle are less affected by ground mineralization than sine-wave detectors. This advantage is counter-balanced by their lower sensitivity to small targets and a lower power-efficiency. The latter necessitates the use of heavy battery packs for portable detectors.
Sine-wave detectors, while capable of high sensitivity, require frequent “re-balancing”, when the soil's or ore's content of magnetic minerals varies. This procedure is unacceptable in stationary industrial metal detectors and it constitutes a major inconvenience for users of portable detectors.
A detector that combined the advantages of both technologies would represent a definite advance in the metal detector art. The realization of this fact has led to attempts to develop detectors that combine the pulse-induction and sine-wave technologies. US Patent Application No. 2005/0062477 by Nelson is an example of this.
While Nelson's patent addresses some of the problems inherent in a dual-technology detector, it does not take full advantage of the opportunities such an approach presents.
In Nelson's patent, the sine-wave and pulse components of the current wave-form are impressed on the coil at the same time. A much more effective result is achieved by separating these components in time.
A practical detector must include means to counteract the influence of the magnetic minerals in the ground. No such technology is described in the Nelson patent. Another method for distinguishing between ferrous and non-ferrous metals is described in U.S. Pat. No. 4,110,679 by Payne. The method relies on the presumption that ferrous metals exhibit hysteresis in their magnetization curves.
The sensitivity of Payne's method is directly proportional to the width of the hysteresis loop. It is known to metallurgists that the process of annealing will dramatically reduce the width of the hysteresis loop. As a result, ferrous targets at the site of a fire, such as nails that were in the walls of a building, may all be erroneously identified as non-ferrous targets. In contrast, annealing will increase the permeability of ferrous materials, which will make them more easily identified by the present invention.
In sine-wave detectors, “ground balancing” is accomplished by placing the demodulating gate so that the portions of the signal above and below the base line cancel, as shown in FIG. 1.
In pulse-induction type metal detectors, the bulk of the ground signal is eliminated by delaying the signal sampling pulse until ground signals have decayed to essentially zero. Unfortunately, this simple expedient allows signals from some desirable targets with short time constants, such as gold nuggets, to also decay to zero before the signal is sampled.
In addition to generating a signal that emanates from the soil and is picked up by the receiver coil, a soil with high magnetic permeability engenders another signal by affecting the mutual inductance between the transmitter and receiver coils.
Unless special measures are taken, this mutual-inductance signal contaminates the signal generated by a target, and its magnitude can be such that the input amplifier of a detector is driven into saturation.
Balanced coil configurations such as the one shown in FIG. 2, mitigate this problem, but owing to the fact the magnitude of the reactive ground signal can be two orders of magnitude larger than the target signal, very precise positioning of the demodulating pulse is required. Often, tiered multi-turn potentiometers are used in the state-of-the-art detectors, to adjust the position of the gate. While touted as means to effect precise adjustment, it is obvious to one skilled in the art that such arrangements are required because the adjustment is very critical.
In a detector using hybrid technology, a better option is available. When the transmitter coil is energized with a linear current ramp, as shown by trace 76 in FIG. 4, the signal sampled at the end of the ramp is only affected by the reactive component of the ground signal. When the duration of the ramp is long enough, all resistive signals have decayed to essentially zero, and thus, reactive and resistive signals have been effectively separated. This is very difficult, if not impossible to accomplish in a sine-wave system.
The linear current ramp 76 generates a DC pulse 78 in the receiver coil. This signal can be cancelled by a compensating pulse of the appropriate amplitude and polarity from a compensating coil, designated 26 in FIG. 3. A negative feedback loop is used to set the digital potentiometer 28 in FIG. 3, so that the signal sampled at the end of the current ramp is driven to zero. When the coil system has been balanced in this fashion, it remains balanced for reactive signals generated after the coil pulse.
In a conventional pulse-induction detector, the target signal is sampled after the transmitter coil current has returned to zero. The eddy currents in the target are induced by the abrupt transition of the coil current. In such a system, the energy contained in the transmitter coil is largely wasted, since a very small fraction is absorbed by the target.
There are systems that attempt to retrieve some of that energy, but methods such as the one described by Candy in U.S. Pat. No. 6,686,742, prolong the transition of the coil current to zero, thus decreasing the voltage induced in the target and reducing the detector's sensitivity to small targets.
In the present invention, the transmitter coil is paralleled with a high-Q capacitor. The energy contained in the transmitter coil is allowed to oscillate until the energy is dissipated in the series resistance of the coil. If care is taken to make the transmitter coil with low-resistance wire and to use a high-Q capacitor, the circuit will “ring” for many cycles.
Thus, additional target signal can be retrieved from each cycle, maximizing the return signal for a given energy expenditure in the transmitter coil.